Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as an energy source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle- or large-sized devices, such as vehicles, use a middle- or large-sized battery pack having a plurality of unit cells electrically connected with each other because high output and large capacity are necessary for the middle- or large-sized devices.
The middle- or large-sized battery pack, which is used as a charging and discharging power source for electric vehicles and hybrid electric vehicles, is manufactured by electrically connecting a plurality of battery modules in series/parallel with each other and mechanically coupling the battery modules to each other. Consequently, the electrical connection between the battery modules is carried out by various kinds of bus bars, such as wires, nickel plates, printed circuit boards (PCB), and flexible printed circuit boards (flexible-PCB).
However, the distances between the connection point of an external circuit and connection points of the battery modules are different from each other due to the structural characteristics of the battery pack including the battery modules connected to each other. Specifically, the difference of internal resistances occurs at the respective battery module connection points due to the difference in length of bus bars connected between the external circuit connection point and the respective battery module connection points, with the result that, when high-voltage current flows, during the charging process and/or the discharging process, the current difference between the respective battery modules occurs.
A middle- or large-sized battery pack for electric vehicles is required to operate for a long period of time, and high-temperature, high-voltage current flows in the middle- or large-sized battery pack. As a result, the difference of the operating conditions, caused due to the small difference of the internal resistance, facilitates the degradation of some battery modules (specifically, the degradation of battery cells constituting the respective battery modules), whereby the overall life span of the battery pack is reduced.
Consequently, some researches have been carried out to solve the above-described problems. For example, Japanese Patent Application Publication No. 2003-346772 discloses circuits (‘connection circuits’) connected between cathode and anode plates of a battery cell and electrode plate connection parts, wherein the connection circuits are constructed in a structure in which the lengths or thicknesses of the connection circuits are changed depending upon the distances between the respective connection circuits such that the connection circuits have the same internal resistance.
In the method of changing the lengths or thicknesses of the connection circuits to equalize the internal resistances, however, the sectional area of the connection circuits is calculated based on the lengths of the connection circuits to design the connection circuits such that the respective connection circuits have corresponding shapes (lengths or thicknesses). As a result, the connection circuits have complicated shapes, and therefore, it is difficult to manufacture the connection circuits, and, in addition, the manufacturing costs of the connection circuits are increased.
Also, during the construction of the middle- or large-sized battery pack, it is required to manufacture different connection circuits which are precisely calculated such that the internal resistances are equalized depending upon the positional change of the battery modules. Furthermore, the internal resistances of the circuits are changed by the temperature. Consequently, the internal resistances of the connection circuits, the lengths or thicknesses of which are previously set as described above, may be changed depending upon the change of the temperature under an actual operating condition, the accurate estimate of which is actually limited to construct the connection circuits. Also, the change in shape of the connection circuits due to the change in design frequently occurs, and therefore, the extensionability and flexibility of the connection circuits are greatly lowered.
Consequently, there is a high necessity for a technology that is capable of fundamentally solving the above-mentioned several problems.